Most modern X-ray hospital equipment not only records images on radiographic plates but also can produce a secondary light image on a scintillation screen which may be viewed directly, photographed with a still or moving picture camera or, as is relevant to the present invention, the secondary light image may be viewed with a flying spot scanner which converts the secondary image into corresponding electrical signals for display on a cathode ray tube, for example. The term flying spot scanners includes inter alia mechanical photoelectric facsimile scanners, television camera tubes of all types, image intensifiers, solid state photoelectric linear and area viewers externally- or self-scanned such as self-scanned photodiode arrays, charge injection devices and charge coupled devices (e.g. Fairchild CCD, Palo Alto, Calif.) and like scanners.
All these flying spot scanners rely on the photoelectric effect, whereas a scintillation screen produces its secondary light image by luminescence. Consequently the dynamic range of the scanners and screen differ considerably. Dynamic range is the ratio of the brightest to the least bright light point of an image source such as the scintillation screen, or the ratio of brightest to least bright light intensity which a flying spot scanner can resolve. The dynamic range of the screen will be influenced by the type of X-ray procedure, the X-ray tube voltage, the dimensions and proportions of bone, tissue and air of the subject. The light from a scintillation screen, taking into account the loss in the lens system projecting the image on the scanner, can vary throughout the image with a dynamic range of 1000. A typical television camera tube for such low light levels has a linear response over a dynamic range of only 100 to 150. Thus a typical camera tube can respond linearly to only a fraction, e.g., one tenth, of the dynamic light range of an available scintillation screen. Obviously detail, resolution and contrast are lost in converting the scintillation image to electrical signals.
Additionally the optical inefficiency of the lens system projecting the scintillation image on the scanner reduces light received by the tube not only generally but particularly by operation of the cosine law which reduces light from the corners of the scintillation screen image most remote from its center. The particular light reduction distorts and reduces contrast in the portions of the electrical signal and display corresponding to the corners of the scintillation image.
Accordingly it is the object of the present invention to provide electro-optical apparatus which reproduces substantially the full dynamic range of light intensity values throughout an image area.